Erosion is a natural process that constantly reshapes the Earth’s surface. It is the mechanism by which weathered material is removed and transported from one location to another. This is distinct from weathering, which is the initial breakdown of rock and soil in place through physical, chemical, or biological means. The energy for erosion comes from natural agents, including water, ice, wind, and the pull of gravity.
Erosion Caused by Flowing Water
Water is the most pervasive and powerful agent of erosion, affecting landscapes from mountaintops to coastlines. River systems, known for fluvial erosion, carve down into the earth primarily through vertical cutting in their upper courses. This downward abrasion by transported sediment creates the characteristic steep-sided, narrow cross-section of a V-shaped valley. The speed and volume of the water dictate its erosive power, allowing it to carry everything from fine silt to large boulders.
At the smaller scale, rainfall initiates sheet and rill erosion on hill slopes. Sheet erosion occurs when a thin, uniform layer of topsoil is removed by surface runoff, often going unnoticed. As the runoff concentrates, it forms small, finger-like channels called rills, which are typically less than 30 centimeters deep. If these rills deepen and widen, they evolve into deep scars known as gullies.
On coastlines, the hydraulic force of waves and currents drives coastal erosion. Waves constantly attack the base of sea cliffs, undercutting the rock and causing the cliff face to retreat inland. This action can lead to the formation of sea caves, which may enlarge to form a sea arch. When the roof of the arch collapses, a solitary pillar of rock, known as a sea stack, remains.
Rivers slow down as they meet a larger body of water, losing the energy needed to transport their sediment load. The accumulation of this fine material at the river mouth creates a delta, a landform that builds new land out into the ocean or lake. The shape of the delta is determined by the balance between the river’s sediment supply and the wave or tidal action of the receiving body of water.
Erosion Caused by Glacial Ice
Glacial erosion is marked by its immense scale and slow, powerful action, transforming mountain ranges and high-latitude landscapes. Glaciers erode the bedrock beneath them through two primary mechanisms: plucking and abrasion. Glacial plucking occurs when meltwater seeps into cracks in the underlying rock, freezes, and expands, wedging out chunks of rock. As the glacier moves, it pulls these loosened blocks away from the bedrock surface.
The embedded rock fragments contribute to glacial abrasion, acting like coarse sandpaper that grinds and smooths the land beneath the moving ice mass. This grinding action creates fine rock flour and leaves parallel scratches on the bedrock surface called glacial striations. This process transforms river-cut V-shaped valleys into the broad U-shaped valleys typical of glaciated regions.
Other landforms are created by this massive ice movement, such as cirques, which are bowl-shaped hollows carved into mountainsides where the glacier originated. The material transported by the glacier, often unsorted and ranging from clay to boulders, is known as glacial till. When this material is deposited at the edges or terminus of the glacier, it forms ridges called moraines, which evidence the ice’s past extent and movement.
Erosion Caused by Wind Movement
Wind erosion, or aeolian erosion, is most pronounced in arid or semi-arid regions with sparse vegetation and dry, loose sediment. It shapes the landscape through two main actions: deflation and abrasion. Deflation is the process where wind lifts and removes loose, fine-grained particles, such as silt and dust, from the surface. This removal can lower the land surface, sometimes leaving behind a closely packed layer of larger rocks that the wind cannot lift, known as a desert pavement.
The second process, abrasion, involves the sandblasting effect of wind-blown sand particles impacting stationary objects. This constant bombardment wears down rock surfaces, creating smooth, polished features or carving rocks into shapes called ventifacts. Over vast distances, wind can transport quantities of fine silt in suspension, which settles to form thick, fertile deposits known as loess.
Erosion Caused by Gravity (Mass Wasting)
Gravity is the ultimate driving force behind all erosion. Mass wasting, also called mass movement, refers specifically to the downslope movement of soil and rock under the direct influence of gravity, without a fluid medium being the primary transporting agent. This process includes a wide range of movements, from the extremely slow to the catastrophic.
The slowest form is soil creep, which is the gradual, barely perceptible downslope movement of soil and loose surface material. Evidence of creep can be seen in features like tilted fence posts, curved tree trunks, or small, low scarps. At the faster end of the spectrum are events like rockfalls and landslides.
A rockfall occurs when individual blocks of rock detach from a steep cliff face and tumble or free-fall to the base. Landslides involve the rapid sliding of a mass of soil or rock along a plane of weakness. These rapid movements are often triggered by an event that overcomes the slope’s internal strength, such as intense rainfall that saturates the ground, adding weight and reducing friction, or seismic activity.